Polycarbosilanes are polymers having a skeletal structure consisting of the elements carbon and silicon, in which Si groups and hydrocarbon groups generally occur alternately. The skeletal structure of such polycarbosilanes consists, for instance, of recurring structural units of the Formula ##STR1## wherein R.sup.0 represents, for instance, a hydrocarbon substituent.
The known preparation methods for polycarbosilanes include two types, one of which starts with monosilanes such as tetramethylsilane, trimethyl chlorosilane, dimethyl dichlorosilane or methyl trichlorosilane, which are converted into mixtures of different polycarbosilanes by thermal decomposition by heating the monosilanes to temperatures of about 700.degree. C. and removing the pyrolysis products from the reaction zone after a short while and cooling them. Preferably, compounds having an Si--C--Si structure are then obtained, the proportion of higher-molecular weight compounds in the pyrolysis mixture increasing with rising temperature and longer residence time in the reaction zone; however, the products prepared by this process are very non-uniform and have low molecular weights.
In the other known process for preparing polycarbosilanes, the starting point is polysilanes which are converted by pyrolysis into the polycarbosilane at temperatures of 350.degree.0 to 450 C. With this process, it is necessary for at least one of the two substituents on the silicon atom in the polysilane which is used to be a methyl group, from which during the thermal conversion process a methylene group is formed which is inserted between two adjoining Si atoms, one hydrogen atom remaining on the silicon atom. Using this process, only those polycarbosilanes can be obtained in which the silicon atoms in the polycarbosilane chain can only be linked by methylene bridges and which can always only have one optional substituent RO desired on the silicon atom, while the second substituent is always hydrogen.
The polysilanes which are to be used for the latter process are obtained by condensation of substituted methyl dihalosilanes in the presence of alkali metals. The pyrolysis of such polysilanes leads to non-uniform polycarbosilanes, or other desirable products cannot be obtained, for instance those which have a second substituent on the silicon atom instead of the hydrogen atom, or those which have a different carbon bridge than the methylene bridge between the Si atoms. During pyrolysis, the already formed polycarbosilane partly decomposes and more or less easily volatile products are formed which, however, are undesirable in the polycarbosilane itself.
The use of a pressure reaction vessel, for instance an autoclave, or an apparatus of the circulation type, which permits return into the circulation, is necessary for the preparation of a polycarbosilane having superior thermal stability and oxidation resistance and a high residual weight ratio after pyrolysis in a non-oxidizing atmosphere by known processes. In the case of a process in which a pressure reaction vessel is used, the reaction has to be carried out for 10 to 15 hours at a temperature of 400.degree. to 470.degree. C. and at a pressure of 81 to 111 bar, and it is absolutely essential to provide pressure-resistant equipment and to take steps against the risk of fire. This process has the additional disadvantage that it is not suitable for mass production. In the case of a process in which an apparatus of the circulation type is used, it is necessary to use an apparatus which contains a heat reaction tower, a product separation tower etc., and low molecular weight products have to be recycled obligatorily in the circulation to the heat reaction tower in order to repeat the reaction. Therefore the temperature must be increased to a considerable extent, to 600.degree. to 800.degree. C., and a long reaction time, e.g. 20 to 50 hours, must be used. The latter process therefore has many disadvantages from the industrial point of view.
Polycarbosilanes in which the Si atoms are linked by bridges of organic aromatic or preferably heteroaromatic radicals such as pyrrol-2,5-diyl or thiophen-2,5-diyl are known from DE-OS 36 34 281. In this case, the object is to prepare conductive polysilanes after additional chemical or electrochemical doping.
It is known according to Schilling and Williams (Schilling, C. L., Jnr.; Williams, T. C. (Union Carbide Corp., Tarrytown, N.Y. USA). Report 1983, TR-83-2; Order No. AD-A141558, 15 pp. (Eng). Avail. NTIS. From Gov. Rep. Announce. Index (U.S.) 1984, 84 (18), 48; see also Chemical Abstracts 101: 196821q) to prepare copolymers in tetrahydrofuran from silane monomer units and olefin units in the presence of potassium. Methyltrichlorosilane, dimethyl dichlorosilane or methyl dichlorohydrosilane as silane monomers are reacted with styrene or isoprene, the Si units being linked by phenyl-substituted ethylene units in the case of styrene. In the case of isoprene, the Si units are linked by the corresponding methyl-substituted C4-alkylene chain, which still has a double bond. In two further examples, isoprene is reacted with methylchloromethyl dichlorosilane or with a mixture of vinylmethyl dichlorosilane and trimethyl chlorosilane.